Spatial Analysis of Cereal Leaf Beetle Abundance in Relation to Regional Habitat Features

Abstract

Environmental correlates of cereal leaf beetle (CLB), Oulema melanopus (L.), adult and egg density in the cereal grain fields of a 41.4 km2 region in Michigan were examined in 1976 and 1977. Discriminant analysis was used to relate high and low insect density to 38 spatially arrayed variables characterizing the fields and the environment within 0.4 km of field boundaries. Habitat features were measured from color infrared photographs taken from an aircraft at high altitude. It was generally possible to identify functions of five or fewer variables that correctly separated 90 to 100% of the fields each year into high or low insect density categories. Variables selected as contributing most to the discriminant functions varied considerably depending on crop, insect stage, and year. The amounts of sparse woods and susceptible (nonresistant) wheat near a field were selected more often than other variables. In only a few cases was an association of high or low CLB density with specific habitat features repeated in both years. Reasons for a general inability to account consistently for spatial variations in insect density in terms of characteristics of fields and the habitat surrounding them are discussed at length. Although localized features of individual fields may be responsible for between-field variation in insect density, these determinants of density are probably many and vary in importance from year to year.